P. tetraphyllum: annual; stems glabrous, much branched, 3-4 inches long, spreading or prostrate; leaves obovate or oblong, really opposite but placed two pairs so close together as to assume the appearance of a whorl of four; flowers very small and numerous, in loose, terminal cymes, the sepals barely a line long; petals much shorter. - South-west coasts. Fl. June, July.